Automatic control for audion amplifiers



July 26, 1932.

AUTOMATIC CONTROL FOR AUDION AMPLIFIERS 4 Sheets-Sheet l S. BALLANTINE Filed Nov. 5. 1927 ...QP l 29 l a r @Mdm-www July 26, 1932. s, BALLANTlNE 1,869,331

AUTOMATIC CONTROL FOR AUDION AMPLIFIERS Filed NOV. 5, 1927 4 Sheets-511691Z 2 TV1 ll E; f @fla 1275/ 4 July 26, 1932. s. BALLANTINE AUTOMATIC CONTROL FOR AUDION AMPLIFIERS Filed Nov. 5. 192? 4 Sheets-Sheet 5 ZZ f-Z'/ www July 26, 1932.

s. BALLANTmE AUTOMATIC CONTROL FOR AUDIN AMPLIFIERS Filed Nov. 5. 1927 4 Sheets-Sheet 4 Imp/,fier

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G1101 nu# Patented July 26, 1932 UNITED sTATEs PATENT OFFICE"- STUABT IBAQLLANTINE, OF MOUNTAIN LAKES, NEW JERSEY, ASIGNOB T0 BOONNN RESEARCH CORPORATION, 0F BOONTON, NEW JERSEY, A CORPORATION OF NEW JERSEY AUTOMATIC CONTROL FOR AUDION AMPLIFIEBS Application led November This invention relates to an automatic control for audion amplifiers, and more particularly to control methods and apparatus for automatically maintaining the output of an amplifier constant in the presence of variations in the carrier Wave energy impressed upon the input system of the amplifier.

In the reception of radio signals, variations in the electric field acting upon the col. lector of the radio receiver arise from natural and fortuitous causes and are commonly called fading. Such variations are-naturally not amenable to control and the irregular variation in the strength of the output is very undesirable in broadcast telephone reception and in other communications. Similarly, in the transmission of signals over land lines by means of carrier Waves, variation in the attenuation may cause marked changes in the carrier Wave ener y which is impressed upon the amplifiers o the transmission line.

In accordance with the present invention, fluctuations in the output are suppressed or eliminated by automatically var ing the rate of amplification of the radio requency or carrier wave amplifiers in such a. way that, as the signal strength increases, the amplification is reduced in amount sufficient to maintain a constant out ut.

In the following speci cation and claims, the term signal strength has reference to the significant input energy which, in the usual carrier wave systems of wired and wireless transmission of signals, isthe car- 'rier wave itself.

An object of the invention is to provide a method of and apparatus for automatically varying the amplification rate of an audion amplifier to suppress fluctuations in out ut due to variations in input energy. Furt er objects are to provide methods of and apparatus for suppressing fluctuations in the output of an audion amplifier by automatically varying the grid bias of one or more amplifying audions in accordance with variations of the input signal energy. More specifically, objects are to provide methods of and means for suppressing the effects of REISSUED fading in radio transmission by varying the amplification of the radio frequency amplifier of a receiver in accordance with variation of the input signal energy.

These and other objects of the invention will be apparent from the following specification when taken with the accompanying drawings in which Figs. 1, 2 and 3 are schematic views illustrating generic forms of the invention,

Figs. 4, 5, 6 and 7 are dia ams showin certain characteristics of au ion tubes, an

Figs. 8, 9, 10, 11 and 12 are electrical diagrams of amplifier circuits embodying the invention.

In the drawings, Figs. 1, 2 and 3 are schematic diagrams representlng the invention as embodied in apparatus for receiving the usual modulated carrier waves employed in radio broadcasting.

In Fig. 1, a conventional radio receiver is represented by the radio frequency amplifier l, the detector 2 and audio amplifier 3. The auxiliary system which provides an automatic control consists generally of a radio frequency amplifier 4, a rectifier 5 and a direct current amplifier 6. The auxiliary system is in shunt with the receiver across the antenna and receives a portion of the incoming carrier-Wave energy, from which is derived a direct current potential which is impressed as a grid bias upon the radio amplilier 1 through the connect-ion 7.

The use of a separate radio frequenc amplilier 4 and also of the auxiliary rectifier 5 1s not essential as the radio amplifier 1 of the receiver and the detector 2 can be arranged to perform the functions of the corres onding auxiliary elements, proper means ing provided to filter out alternating current components before impressing the output jupon the direct current amplifier or upon the grids ofthe radio frequency amplifiers. The resultant reduction in the parts required 5, 1987. Serial No. 231,878.

-for the auxiliary system is of considerable economic advantage in certain practical4 applications of the invention.

The diagram of Fig. 2 represents a radio receiver suchV as shown in Fig. 1, and an auxiliary system comprising the rectifier 5 and direct current amplifier 6, the rectifier receiving amplified signal energy from the output of the receiver radio frequency amph- The diagram of Fig. 3 represents a further simplification of the general arrangement o f Fig. 1, the direct current amphier 6 in this case being connected across the detector The method of operation will be described in connection with the system shown 1n Fig. 2. The curve of Fig. 4 is a gra hical re resentation of the variation in t e ampli cation in a four-stage one-way amplifier of the type shown in my copending application Serial No. 629,702, filed April 3, 1926, as the bias of the ids varied through 1positive and negative va ues. The values of g represent the voltage across the intput o f detector 2 for varying values of Ec, the direct current bias on the grids of the radio frequency amplifier audions. The relationship may e expressed mathematically as:

Eg=A(Ec)Eo (1) where Eo is the impressed voltage due to the wave field or incomingusignal energy.

The grid bias is a notion of t e voltage Eg, the precise formvof this function being determined by the characteristics of the rectifier 5 and direct current amplifier 6; that is EG=(E9) (2) The exact values attained b Eg'in view of the simultaneous action ofluations (l) and (2) may be found graphica y as shown in Fi 5. The intersections of the curves give t e values of outputyoltages Eg, corres onding` to the various impressed voltages, yielding the Fig. 6 solid line curve of input Eo versus output Eg for the whole system:

Ideal operation would give the broken line curve of Fig. 6, i. e., an increase in input energy would produce a corres onding increase in out ut until a critica output Eg is reached); the output not increasing thereafter with any increase in input energy; This ideal relationship would obtain if t e rectifier action was such as to furnish the relationship between positive lgrid bias Ec and rectifier input Eg which is shown by the dotted line in Fig; 5. Such a rectifier is diicult to obtain in practice 'for it demandsthe characteristic shown in dotted line 'in Fig. 7 that is, one 'in which there is no output until 'the im ressed volta Eg increases to a critical `va ue E at whic the output suddenly rises. It is possible, however, to attain a practical approximation to this requirement, as shown by the, so 'd line curve of Fig. 7, which gives satisfactory operation in accordance with this invention.

None of the known rectifier-s possesses a critical region for low impressed yoltngeamplitudes as all have a region of transition 'iyhere it was customary tof operate in the prior art on account of the necessity for lar e curvature with the small signals then available. It is possible, however, as described in the copending a lication of Ballantine and Hull, Serial o. 644,215, filed June 8, 1923, patented 'January 8, 1929, No. 1,698,668, to o tain a close approximation to discontinuous action with certain rectifiers by first amplifying the signals sufficiently; the amplification being of an order materially higher than that obtained in the usual practice, i. e., say from ten to one hundred times that ordinarily empio ed.

A characteristic such as s own in solid line in Fig. 7 may be obtained with commercial ferro-silicon alloys containing about to 80% silicon and about 30% to 20% of iron, the contact point being iron pyrites (FeSz) held in light contact with a sensitive spot in the ferro-silicon. A rectifier of this type will possess a critical point E which, when exceeded, will bring the rectifier into action. The critical point may be adjusted to secure various constant outputs. It will be noted that the response above the critical value is substantially linear, and this feature is particularly desirable in the systems of Fig. 3 to avoid distortion, but is not necessary in the systems of Figs. 1 and 2 so long as the rise of t e characteristic curve above the vertical voltage is suiiiciently steep.

Therniionic rectifiers of suitable design ma be employed in place of the ferro-silicon an ironpyrite or ferro-silicon and` metallic contact rectifiers. I have found that the lack of a sharp cut-off in the case of thermionic devices is due largely to the circumstance of the applied voltages being near the same order of magnitude as those corresponding to the emission velocities of the electrons, energy due to thermal agitation and the voltage corresponding to the work function of the metal, and also to the Maxwellian distribution of the velocities of the electrons. Without increasing the applied volta e, somewhat better action may be obtained y decreasing the cathode temperature and providing a metallic vapor of low work-function, such as caesium, which emits copiously at low temperatures,

.but in general the preferred method of obtaining a sharp cut-o is by raising the applied voltages.

Referring again to Fig. 4, it will be seen the controlling grid bias may be either positive or negative and that on the positive side the control is somewhat better for a given change in Ec than on the negative side. The positive control, while more effective, is under some disadvantage due to the grid current which is permitted to How in the radio frequency amplifier tubes. This extra conductance decreases the selectivity and may in some cases lie found intolerable; in these cases the negative control may be used, the extra ran e of Ec required being obtained by suitable irect IFS lll)

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current amplification or increased rectifier eflicienc 4 Practical embodiments of the invention as applied to radio receivers are shown in the circuit diagrams of Figs. 8, 9 and 10.

The circuit of Fig. 8 is an embodiment of Y the schematic arrangement shown in Fig. 3,

but omitting the direct current amplifier of that system. The radio frequency amplifier unit 1 may be of any suitab e design and is indicated aslcomprisxng a plurality of audions 10 which with their interstage couphng and tuning devices are arranged within a grounded shield 11. The grid returns of the several sta are preferab y correspon ing filament circuits b radio-frequency by-pass condensers 12, an the several grid returns upon which the variable bias is to be impressed are connected to the lead 7 from the automatic control unit.

The detector unit 2 of the receiver serves also as the rectifier unit of the control system, and is coupled to the amplifier unit 1 by a transformer 13. The detector 14 is preferably of the ferro-silicon and iron-pyrite type,

, and is connected across the transformer 13 through the bias battery 15, by which the critical voltage of the rectifier 14 is adjusted, and thev resistor 16 which serves as a coupling for the audible alternating current components of rectification. The audio fre uency amplifier unit 3 is connected across t e lresistor 16; a variable tap being provided for regulating the alternating current input.

To rovide the variable direct current bias free rom alternating current components, the resistor 16 is by-passed by condenser 17 of low radio f uency impedance; and a filter comprising t e resistor 18 and condenser 19 is also connected across the resistor 16, the variable bias lead 7 being brought to the junction of filter elements 18 and 19. The impedance of the condenser 19 and other capacities in parallel with it is low for the audio frequency components. A battery 20 may be provided` in the variable bias lead 7 to rovide the normal grid bias in the absence of a signal, but this battery may often be omitted. The diagram of Fig. 9 shows an alternative arrangement of the rectifier circuits. The amplifier unit is not shown in this diagram but may be of the type shown in Fig. 8. An isolating condenser 21 is arranged between the detector 14 and the input transformer 13 to prevent short circuiting of the detector output circuit by the transformer secondary. The detector output includes the bias battery 15 and the resistor 16, the audio frequency unit3 being coupled to the resistor 16 through a variable tap. This connection may include an isolating condenser 22 when the direct currentA drop across resistor 16 due to the battery 15 does not provide the proper grid bias for the first tube of the audio amplifier. Under these circumstances, the grid leak 23 is connected to the' i pedance,

emplo ed with condenser 22 to provide the desire bias.

It will be a arent that a thermionic diode (two electrodiig tube ma -be substituted for the crystal rectifier the laments being usually connected to the ground.

The circuit of Fig. 10 illustrates the generic circuit of Fig. 3, the rectifier in this case being a thermionic triode. The radio freuency amplifier is not shown and the circuit e ements which are substantially identical with those 'of Figs. 8 and 9 are identified by the same numerals. The radio frequency transformer 13 supplies the thermionic detector 24 through the isolating condenser 21, the detector grid being biased by a battery 25 through the grid-leak 26. The alternating current components of the output is transferred to the audio frequency unit 3 by a low frequency coupling transformer 27.

yThe plate circuit also includes a plate battery 28 and a resistor 29 for passing the direct current component to the direct current amplifier unit 6. The resistor 29 is shunted by a condenser 30 of low audio frequency imwhich condenser keeps the alterna t ing components from the direct current amplifier and also prevents the resistor 29 from interfering with the operation of audio transformer 27 at audio frequencies. The detector plate battery 28 may be located between the resistor 29 and the filament as well as in the position shown. The grid bias for the direct current amplifier 31 may be obtained from the potential drop across resistor 29 due to battery 28, by means of a separate bias battery 32,l or by equivalent means. As shown, the plate circuit of the direct current amplifier is supplied by a separate battery 33, but it is obvious that this may be common with the other plate batteries. The direct current output is coupled to the grid returns by means of resistor 34, the lead 7 including a battery 20, if desired, for adjustment of the mean bias.

The tetrode or four electrode tube is a very convenient form of direct current amplifier as it ryields a considerable amplification with low battery voltages. The method of substituting the tetrode for the triode shown in Fig. 10 will be obvious to those familiar with audion circuits.

The circuit diagram of Fig. 11 represents another embodiment arranged for negative control, that is, control by means of a negative bias on the radio frequency amplifier 1. The system is of the general type shown in Fi 2 the radio receiver comprising the ra io frequency amplifier 1, detector 2 and audio amplifier 3 and the control system comprising a separate rectifier 5 in parallel with detector 2, and a direct current amplifier 6. The rectifier element or diode 35 is connected across the transformer 13 through isolating condenser 36, and the output circuit of the rectifier includes a radio frelpency choke 37, battery 38 and resistor 39 w ich may be about 25,000 ohms. The resistor 39 serves as a couplin to the direct current amplifier unit 6 an a by-pass condenser 40 is shunted across the battery 38 and resistor 39 to keep radio frequency currents from am lifier 6; the choke 37 preventing the con enser 40 from shorting the rectifier for radio frequency voltages. The battery 38 provides a means for ad] usting the critical voltage of the rectifier 35.

The direct current amplifier element is shown as a tetrode 41 which has its control grid cou led to the rectifier output through a grid bias battery 42. The usual batteries 43, 44 supply the operating potentials and the output is developed in a resistor 45 of about 100,000 ohm resistance which is shunted by a condenser- 46 of low reactance for audio modulation frequencies. The variable bias lead 7 is connected to resistor 45 by an adjustable tap, and preferably includes a battery 20 for placing a normal bias on the grids of the radio frequency am lifier 1.

It will be evident that this arrangement may be used for positive control, i. e., with the radio frequenc bias varying positively instead of negative y, by omitting the direct current amplifier and connecting the bias lead 7 to the resistor 39 instead of to resistor 45 as shown. y

The circuit of Fi 12 illustrates an analogous arrangement ut adapted for positive control. The symbols are for the most part uniform with those of the preceding figure and the parts have the saine functions. In this case the filaments of the rectifier and drect current amplifier tubes are at ground potential with those of the radio fr uency amplifier, thus permitting the use o coinnion A batteries. Also a triode has been shown instead of the tetrode in the direct current amplifier. that if the output tap is shifted from resistor 45 to resistor 39 the control changes from sitive to negative.

Although the use of a rectifier having a sharp cut-o, and in some cases having a substantially linear characteristic above the cutoff voltage, is particularly desirable for close regulation and good quality, it will be apparent that some improvement in volume control may be had with rectifiers having other characteristics.

In general the use of a separate rectifier for control purposes is prefera le to using the regular detector of poses. The reasons for this have been given. A goed combination is a linear detector at 2, and a linear rectifier at`5 with a cut-off characteristic. A linear rectifier at 5 without a cut-ofi` characteristic is less useful for control purposes with a characteristic of the type shown in Fig. 4. The variable bias may be It will also be obviousthe receiver for both pur' placed upon the detector stageor audio amplifiers in addition to or in place of the bias on the radio frequency amplifiers.

It will be ap arent .that the invention is capable of wi e application with various types of audion amplifiers and that many circuits may be devised for effecting control by the principles which are described in this specification.

I claim:

1. An audion amplifier including an audion for signal wave amplification, an output circuit, and means sup ressing fluctuations in said output circuit ue to variations in the strength of an incomin signal, said means comprising a rectifier 01' incoming signal energy, and means for impressing upon the grid of said audion a direct current bias volta e derived from said rectifier, said rectifier being of the type having an approximately linear relation between direct current output and radio frequency input above a critical input voltage.

2. In combination, an audion, an output circuit therefor, and means for suppressing fluctuations in said output circuit due to variations in 'the strength of an incoming signal, said means cdinprisingl a rectifier having a critical input voltage below which substantially no direct-current out ut is obtained, and means for adjustingsaid critical voltage.

3. In an electrical system, the combination with an audion having input and output.

circuits, of means for suppressing fluctuations in said output circuit due to variations in the strength of an incoming signal in said input circuit, said means comprising a. rectifier of the type having an approximately linear relation between direct-current outut and radio frequency input above a critical input voltage, and means for varying the critical voltage of said rectifier.

4. The combination with a receiving system comprising an audion, of means energized by said audion for impressing a variable bas on said audion, said means including a rectifier, and voltage-biasing means for said rectifier. i

5. The combination with an audion, of means for biasing said audion by a directcurrent voltage, means for automaticall varying said bias voltage in accordance wit the strength of an incoming signal, at least one of said means including a 'rectifier having a critical working voltage, and means whereby said critical voltageinay be adjusted.`

6. In a stem for the transmissionL o modulated signals, the combination with an audion amplifier for amplifying said signals, and means automatically contollingi'the gain of said amplifier in accordance with the strength of received signals, of a demodulator, and means to suppress all output from iis necesa: Y5h4 impressed thereon reach a predetermined critical value.

7. The invention as set forth in claim 6, wherein said last means is adjustable to con- 5 trol the predetermined value of amplified signal strength below which no demodulator output is obtained.

8. In a receiver for modulated carrier waves, the combination with a signal fre- 1 quency amplifier, and a demodulator having an ,audio frequency response substantiall proportional to the strength of impresse signal volta es of a linear rectifier for rectifying ampli ed signal voltages to produce a direct current voltage for automatically controlling the ain of said amplifier, and means for suppressin the direct current response of said rectifier or impressed signal voltages below a predetermined value.

9. The invention as set forth in claim 8, wherein said means is adjustable to control the predetermined amplified signal value below which no gain control voltage is developed bysaid rectifier.

10. In a receiver for modulated carrier waves, a signal frequency amplifier, means for producing audio frequency and direct current voltages by rectification of amplified signals, means actuated by said direct cur- 3 rent volta e to control the gain of said amplifier, an means suppressing the development of audio frequency and direct current voltages for im ressed amplified voltages below redetermmed values.

11.' he invention as set forth in claim 10, wherein said rectification means has a linear response for impressed signal voltages above the predetermined value. f

12. The combination in a receiver for car- 40 rier wave signals, of an audion amplifier, a linear rectifier upon which the am lified si nal voltage is impressed, means inc uding said rectifier for im ressin a variable gaincontrol bias on saidJ ampli er, said rectifier 45 having a critical input voltage below which no rectified output obtains, and means for ad'usting said critical input voltage.

n testimony whereof, affix my signature.

STUART BALLANTINE.

DISCLAIMER 1,869 331.-Stuart Ballantine, Mountain Lakes, N. J. Aan'roiii'ric CONTROL Foa UnioN AMrLIriiias. Patent dated July 26, 1932. Disclaimer filed October 12, 1933, by the assignee, Boonton Research Corporation. u Hereby enters this disclaimer to the subject matter comprising claim 1, which claim 1 reads as follows: 0

1. An audion amplifier includin an audion for signal wave amplification, an output circuit, and means suppressing uctuations in said output circuit diie to vaiiations in the strength of an incoming signal, said means comprising. a rectifier for incoming signal energy, and means for impressing upon the grid of said audion a dir ect current bias voltage derived from said rectifier, said rectifier being of tliety e having an approximately linear relation between direct current output and radio requency input above a critical input voltage.

[Oficial Gazette November 7, 1933.]

necesa: Y5h4 impressed thereon reach a predetermined critical value.

7. The invention as set forth in claim 6, wherein said last means is adjustable to con- 5 trol the predetermined value of amplified signal strength below which no demodulator output is obtained.

8. In a receiver for modulated carrier waves, the combination with a signal fre- 1 quency amplifier, and a demodulator having an ,audio frequency response substantiall proportional to the strength of impresse signal volta es of a linear rectifier for rectifying ampli ed signal voltages to produce a direct current voltage for automatically controlling the ain of said amplifier, and means for suppressin the direct current response of said rectifier or impressed signal voltages below a predetermined value.

9. The invention as set forth in claim 8, wherein said means is adjustable to control the predetermined amplified signal value below which no gain control voltage is developed bysaid rectifier.

10. In a receiver for modulated carrier waves, a signal frequency amplifier, means for producing audio frequency and direct current voltages by rectification of amplified signals, means actuated by said direct cur- 3 rent volta e to control the gain of said amplifier, an means suppressing the development of audio frequency and direct current voltages for im ressed amplified voltages below redetermmed values.

11.' he invention as set forth in claim 10, wherein said rectification means has a linear response for impressed signal voltages above the predetermined value. f

12. The combination in a receiver for car- 40 rier wave signals, of an audion amplifier, a linear rectifier upon which the am lified si nal voltage is impressed, means inc uding said rectifier for im ressin a variable gaincontrol bias on saidJ ampli er, said rectifier 45 having a critical input voltage below which no rectified output obtains, and means for ad'usting said critical input voltage.

n testimony whereof, affix my signature.

STUART BALLANTINE.

DISCLAIMER 1,869 331.-Stuart Ballantine, Mountain Lakes, N. J. Aan'roiii'ric CONTROL Foa UnioN AMrLIriiias. Patent dated July 26, 1932. Disclaimer filed October 12, 1933, by the assignee, Boonton Research Corporation. u Hereby enters this disclaimer to the subject matter comprising claim 1, which claim 1 reads as follows: 0

1. An audion amplifier includin an audion for signal wave amplification, an output circuit, and means suppressing uctuations in said output circuit diie to vaiiations in the strength of an incoming signal, said means comprising. a rectifier for incoming signal energy, and means for impressing upon the grid of said audion a dir ect current bias voltage derived from said rectifier, said rectifier being of tliety e having an approximately linear relation between direct current output and radio requency input above a critical input voltage.

[Oficial Gazette November 7, 1933.] 

